A compact wireless communication equipment such as a cellular phone has a built-in compact antenna device. FIGS. 18 and 19 are schematic perspective views showing examples of configuration of a conventional antenna device.
An antenna device shown in FIG. 18 includes a substrate 1 constituted by a dielectric rectangular cuboid and a linear radiation conductor 2 that is formed at a center portion of an upper surface of the substrate 1 in a lateral direction. One end of the radiation conductor 2 is connected to a feed electrode (a feed line) 4 via a gap g, and the other end of the radiation conductor 2 is connected to a ground conductor 3 that is formed on a bottom surface of the substrate 1. Because an open end of the radiation conductor 2 is electromagnetically coupled to the feed electrode 4 via a capacitance of the gap g, it can be excited in a non-contact manner with the feed line, and even when the antenna device is downsized, an impedance matching can be easily obtained (see Japanese Patent No. 3114582).
In the antenna device shown in FIG. 19, the other end of a radiation conductor 2 is bent in an L shape. A feed electrode 4 is formed on a substrate 1, which is connected to a short end of the radiation conductor 2 via the gap g. With this configuration, it is possible to increase a resonant wavelength of the antenna with respect to a chip size.
However, because the conventional antenna device shown in FIG. 18 has a structure in which ends of elongated linear conductor patterns face each other across the gap g of a predetermined width, and a range of facing each other is narrow, resulting in a problem that it is not possible to obtain a large capacitive coupling.
On the other hand, in the conventional antenna device shown in FIG. 19, because the feed conductor is formed along the longitudinal direction of the radiation conductor, the range of facing each other is relatively wide, so that it is possible to obtain a larger capacitive coupling. However, because the feed conductor is formed on both top and side surfaces of the substrate, it is necessary to secure a wide area for forming the feed conductor. Therefore, it is not possible to utilize a principle surface of the substrate in an efficient manner for the radiation conductor, resulting in a problem that the size of the entire antenna device is increased.
Further, the antenna devices shown in FIGS. 18 and 19 have a problem that the antenna characteristics are largely changed depending on a mounting position on a printed circuit board. This problem is mentioned in Japanese Patent No. 3331852, which describes that the antenna characteristics are changed depending on a change of a positional relationship between a ground pattern on the printed circuit board and the antenna device.
A phenomenon that the antenna characteristics are changed depending on the mounting position becomes prominent when the radiation conductor and the feed conductor are capacitively coupled using a gap. Therefore, to suppress the change of the antenna characteristics depending on the mounting position, it appears that the radiation conductor and the feed conductor should be coupled to each other in a method other than the capacitive coupling.